1. Field of the Invention
This invention generally relates to the field of paper box folding and sealing and more specifically to an improved apparatus and method for providing a box with a sealed, hemmed edge.
2. Description of Related Art
This invention has particular application to paper box folding and sealing apparatus that moves a succession of carton blanks along a paper line to form, individually, open ended boxes, generally in a folded form. Although carton blanks are formed from a wide variety of materials, this invention is generally related to the manufacture of folded boxes from carton blanks having surface coatings of a thermoplastic material. Such carton blanks generally are laminated with a paper board core and outer coatings of a high or low-density polyethylene or other thermoplastic material. The laminate may contain an optional barrier of metallic foil or other material that forms an essentially gas impermeable membrane.
In some applications a box formed from such a laminated carton blank stores only dry contents or stores materials for very short time intervals. A conventional paper box folding machine for folding such boxes merely overlaps the longitudinal edges of a laminated carton blank, locally heats these overlapped edges and presses them together so the thermoplastic coatings cool and form a sealed longitudinal seam that completes the box in its folded form and in any expanded form. This longitudinal seam should be strong because any failure along this seam can destroy the integrity of the box. It is also important that the seam be formed on paper box folding machines without reducing the operating speed as any such reduction usually increases the box manufacturing costs.
In other applications a box may be designed to store liquids or to store contents in a moist environment. Merely overlapping the edges of the carton to form the box does not produce a longitudinal seam that maintains its integrity over an extended time interval for such applications. Specifically, the conventional overlapped configuration exposes one edge of the paper board core that is at the center of the laminate. If that edge lies inside the carton, the liquid contents can wick through the paper board core and eventually destroy the carton. If that edge lies outside the carton, moisture from the external environment also can wick into the paper board core and can even enter into and contaminate the carton contents. Consequently, under the foregoing conditions it also is important the folded carton prevent such wicking in order to maintain the integrity of the longitudinal seam.
Several sealing solutions have been proposed to provide a strong, reliable longitudinal seam for forming such boxes. According to one proposal, a sealing material is applied to the exposed edge. This process, however, does not readily adapt to conventional paper box folding machines. It is difficult to control and can reduce machine operating speed significantly.
In accordance with another approach, the paper box folding and sealing apparatus forms simultaneously a sealed hem and the longitudinal seam. U.S. Pat. No. 3,495,507 (1970) to Haas et al, for example, discloses apparatus that skives a strip of carton material adjacent to the edge of a sealing panel, scores the center of the skived strip and then folds the skived strip on itself to form a hem. The hem then is heated and pressure bonded to an overlapped portion of the carton blank to form the sealed hem and longitudinal seam simultaneously. In this hem the skived faces are in intimate, but unsealed, contact.
U.S. Pat. No. 3,654,842 (1972) to Schwank discloses a similar structure that overcomes an apparent problem in Haas. Specifically as the folded, but unsealed, hem moves along a paper line as shown in Haas et al, the hem can open. Schwank minimizes this effect by cutting or forming a score line down the center of the skived portion.
U.S. Pat. No. 3,754,456 (1973) to Andrews et al discloses other apparatus for folding a foil about the edge of a paper board blank to provide a seal. In these embodiments, however, the carton leaves an exposed edge at the longitudinal seam on the exterior of the carton that is subject to wicking.
Apparatus according to these disclosures also can limit maximum machine operating speed. Andrews et al, for example, discuss maximum operating speeds in the range of 400 feet per minute. Apparently there is an inability to control the folding process accurately at elevated speeds. The systems such as disclosed in Haas et al and Schwank operate at higher speeds but generally less than 1000 feet per minute. In many cases this limit controls the maximum machine operating speed.
It also has been suggested to separate the manufacturing steps for forming a sealed hem and for forming the longitudinal seam. For example, U.S. Pat. No. 3,604,317 (1971) to Braun discloses apparatus that makes a partial cut through the blank parallel to an edge to be hemmed. Skiving then removes a significant portion of the material between the outer edge and the cut including the thermoplastic material and a portion of the core facing the skiving apparatus. After the skived area and the adjacent area pass over an open burner, folding fingers wrap the skived portion around the end at the cut to lie against a lower coated carton surface adjacent the skived portion and form a hem. Pressing the skived portion against the coating adjacent the cut produces a bond with the skived portion to complete the hem. Braun recognizes that aerodynamic forces acting on the blanks become important considerations at higher operating speeds, specifically relative to the skiving operation. However, as the aerodynamic effects become more prevalent, it also becomes more difficult to accurately engage the flap after it leaves the burner. Consequently, elevating speed increases the difficulty encountered in folding the skived portion through 180.degree. onto the adjacent portion of the carton in an accurate and repeatable fashion.
Further, if the heating section of the apparatus is not carefully controlled, the heat can raise the coating temperature on portions of the carton adjacent the hem that are not involved in sealing the hem. At such temperatures, polyethylene can accumulate on subsequent apparatus thereby changing the operation parameters of that apparatus and require unnecessary cleaning that interrupts manufacture. Moreover, if these coatings are located at the longitudinal seam, the subsequent longitudinal seal may be weakened. These considerations still impose an operating speed limit of about 800 to 1000 feet per minute.
U.S. Pat. No. 4,540,391 (1985) to Fries, Jr. discloses another method and apparatus for skiving and hemming paper board boxes. Two successive skiving operations produce an approximately flat outer skived surface parallel to the surfaces of the carton and a sloped inner skived surface. After the skiving operation is complete, the outer skived portion folds over the sloped skived portion and adjacent portions of the carton and apparatus irons the outer skived portion in place. This allows the outer skived portion to spring into an upright position at about 90.degree. to the plane of the carton after the ironing section is complete due to internal restoring forces within the carton. A heater raises the temperature of both the skived portions and the thermoplastic coating adjacent the skived portion to a level below which the exposed paper board in the skived portion scorches. After the blank leaves the heating station, the upright skived flap is folded onto the carton blank before it passes through a clamping or sealing structure that compresses the overlapped skived portion against the carton blank while the thermosetting material cools.
The resulting hem has an unsealed interface between the sloped skived portion and the abutting flat skived portion. These surfaces can move relative to each other and constitute a potential source of hem and longitudinal seam weakness. The dependence on internal restoring forces to retain the skived portion to a generally upright position can produce a significant range of angles particularly as speeds increase. At Fries' permissible upper temperature limit it is possible for sufficient heat to transfer through the skived portion to activate the thermosetting plastic on the remote side of the carton thereby potentially reducing the effectiveness of the longitudinal seam. As previously discussed, the thermoplastic material can accumulate on the following folding apparatus at these temperatures. These and other parameters of the Fries system apparently limit the operating speed of the folding and sealing apparatus to about 600 to 1000 feet per minute.